Jason Rowley is a venture capital and technology reporter for Crunchbase News.

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Since the earliest days of the commercial web, there was a Faustian bargain struck between users and content publishers: Users got free access to online content in exchange for tolerating ads sold by the very same publishers delivering the latest words, images, and videos.

And, by and large, it’s a bargain that’s worked out well for digital advertisers.

According to data published on Statista, combined global digital advertising spending is expected to be around $229 billion (USD) in 2017. By 2020, that’s projected to grow by over 45 percent, to about $335.5 billion.

In many industries, that kind of growth might point to new kinds of market opportunities ripe for the picking by fast-moving startups. But the concentration of earning power in just a few giant companies, plus other market forces, leads us to the conclusion that venture capital investment into advertising technology startups has already peaked.

In the five years elapsed since the peak of advertising technology deal-making activity in H1 2013, the pace of venture capital investment has declined by nearly 80 percent.

The advertising industry is multifaceted, with a dizzying array of companies serving almost every conceivable niche in the business. That said, even some of the more promising categories of advertising tech have likely seen their peaks come and go.

While there isn’t enough raw data about ad platforms and advertising networks, or local and mobile ad technology companies, to draw meaningful conclusions, we can combine these subcategories into two groups. By plotting the proportional distribution of deals struck at the beginning of 2012 and the end of 2016 between the two groups, we can see that these once-bright categories might be has-beens.

(Note: Other promising categories like social media advertising and video ad platforms had insufficient data for this analysis.)

At least within the last five years, both groups had reached local maxima a while ago. If every year were equal between the half decade, we’d expect to see a flat line at 20 percent. But of the set of deals counted for companies in the local and mobile advertising categories from the last five years, 27.2 percent of those were originated in 2014. Ad platforms and network startups found their peak deal counts just one year later, in 2015, when approximately 26.4 percent of those deals were struck.

Of course, 2016 was a difficult year for VC investment in almost all sectors, so it’s possible that there will be some kind of recovery from those declines. With that said, the consistent downtrend in adtech investing activity depicted in the first chart might suggest otherwise.

Concentration of (earning) power

“The best minds of my generation are thinking about how to make people click ads,” said Jeff Hammerbacher, then an early employee at Facebook, in an interview with Ashlee Vance for Bloomberg Businessweek.

The result of all that perseverating? A head start in an industry where network effects are literally the root of all value. According to one slide in Mary Meeker’s 2017 Internet Trends Report, Google and Facebook, taken together, earned around two-thirds of US Internet advertising revenue in 2016.

This leaves the remaining third of US Internet advertising revenue to everyone else, which includes smaller competitors in the form of social networks like Twitter and Snapchat, native advertising and media companies like BuzzFeed, tools for publishers like Taboola and Outbrain (which are responsible for those wacky ads at the bottom of many news articles), and a long tail of others.

But even with all that brainpower in ad tech, it’s still possible to run up against barriers.

Stop following me

Over the past several years, trends have converged to encourage an increasing number of web users to install ad-blocking software. That main trend is the ever-more annoying types of advertisements that have crept onto desktop and mobile phone screens.

According to findings by PageFair, a provider of adblock analytics and, somewhat perversely, “Adblock-proof ad serving,” some 600 million devices were running adblock software globally at the end of 2016. Of those devices, 62 percent were mobile. That same report indicates that there was a 30 percent year-over-year increase in the usage of ad blocking software.

This trend toward increased ad blocker usage has been taken up by several browser makers. Apple’s Safari browser, which has approximately 3.6 percent of the world’s market share, will ship with features that block web trackers and auto-playing video content. And Google’s Chrome browser, which controls nearly 60 percent of the global browser market, will implement a new advertising filter.

Although there seems to be a constant arms race between ad developers and those building the ad blockers, it is almost certain advertising giants are likely to weather this storm longer than some scrappy upstarts.

The ad in the high castle

Facebook and Google have built quite the moat for themselves in the form of our data.

Looking just at Google and Facebook, both are well-positioned to continue to dominate the digital advertising industry precisely because these companies reach so deeply into each of our lives.

In a big way, Facebook owns a digital model of our communication channels and social relationships, all modeled by and fed into the social graph. And Google owns the browser, email communications, video consumption habits, and, most importantly, over three-quarters of daily search engine traffic.

And in the case of both companies — Facebook with Instant Articles and Google with its Accelerated Mobile Pages (AMP) project — there’s the very real possibility of separating web content from publisher-owned websites, which, at least on mobile, cuts out the startups that have heretofore made the internet money engine run.

For entrepreneurs in the advertising space, market opportunities do and will continue to present themselves, but the structure of the market is such that the lion’s share of the revenue growth will go to a small handful of incumbents, leaving relatively slim pickings for the rest.

It’s no wonder that, at least for now, VCs are increasingly shying away from advertising technology investments. The moat just keeps growing wider.